Pool covers should be used during your swimming season. If you use your pool during the daytime, take off the cover just before swimming and replace the cover as soon as you're done using the pool.
If you use your pool only at night, the effectiveness of a pool cover will depend on whether the evaporation and other losses prevented by the cover exceed the solar gain reduction caused by the cover. The type of cover and the climate affects this balance. In dry and/or windy conditions, the evaporation rate of the pool increases. Therefore, it is generally beneficial to have a transparent or solar bubble cover on during daylight hours. In warm, humid conditions the evaporation rate decreases. In this case, it may be more beneficial to leave the cover off during the daytime.